Furnace control system



March 22, 1938. E. M. cLAYToR 2,112,067

` FURNACE CONTROL SYSTEM `Filed April 14, 1954 wlTNEssEs Patented Mar.v2 2,1938 o o t It UNiTEDsTAT-Es PATENT lOFFICE FURNACE CONTROL- SYSTEM Edward M. Claytor, Wilkinsburg, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application April 14, 1934, serial No. '120,668

2 claims. (c1. 15s- 1) My invention relates to systems of control for the line contactors 8 and 9 in a manner explained furnaces and more particularly to a follow up more clearly hereinafter. system having anti-hunting ffeatures for con- If the steamA pressure 'in the boiler is to'be trollingthe operating characteristics of a motor maintained at a given value, but because of the or a plurality of motorscontrolling the operation excessive demand, or for other reasons, the pres- 5 of a furnace. sure drops and it is necessary to increasethe fuel With thefollow-up systems heretofore utilized, supplied, in such case it is also necessary to in-l considerable diflicultyis experienced because of crease the draft to thereby maintain the efthe fact` that these systems have the habit of ciency of combustion at or near the desired value.

10 oscillating or hunting when a speed change o1 `If the fuel supplied from the pipe 2 is increased 10 the motor to becontrolledis to be effected. If by appropriate manipulation of the throttle valve theinduced draft for the furnace of a boiler is 25, the speed ofthe motor 3 must be increased to be controlled and the fuel supply is changed acorresponding amount, and this result is accomv -or, if the battery of Stoker motors are to be conplished by appropriate operation of the rheostat l5 trolled in relation "to the` pressure in the iiue, 5 by the motor 6. If, however, the `motor 6 were 15 hunting vcauses an improper operation and often calledupon to effect the complete correction` at a wastage of fueland a lowering ofthe efficiency one operation, it would invariably overshoot so of the plant. ,l that` a reverse operation is necessary and hunt- One object of my invention is to. provide, in a ing would `take place. To eliminate this undecontrol for a furnace, a follow-up systemwheresirable feature, the time-limit contactors I0 and l in hunting'is substantially.eliminated II are provided to control the line contactors 3 Other objects and advantages of my invention and il.` This controlling action will be better unwill become apparent from a study of the followderstood from a study of a typical sequence vof ing specification, when considered in conjuncoperation. y

5 tion with the accompanying drawing, wherein If the ,attendant actuates valve 25 to supply the` single. figure illustrates diagrammatically a more fuel to the furnace, the pressure in adjasystem of control, namely'a follow-up system, for cent chamber 26 rises and, in consequence, the controlling the speed ofk a motor operating a fan pressure in cylinder 21 risesl forcing the piston or blower for inducing a draft for a boiler fur- 2B upwardly against springs 'I2 and 13. The

.30 nace, t junction 29 of the floating lever 1I, to whichthe 30 Referring more particularly to thegure, a rod of piston 28 is attached, is thus moved upboiler and 'furnace I is provided with a fuel supwardly because, for the assumptions made, pivot ply pipe 2 which, in this instance, is an oil supply point 30 of lever 1I with the rod of piston 49 in pipe, and a motor 3`for driving a draft fan discylinder 48, remains stationary. The position posed in the housing 41. The motor armature ,of pivot point 30 may be adjusted by the spring 35 I3 may be energized from buses ,I2 and I4. `'The assemblies 12 and 13, respectively, so that any motor 3 is provided with a eld winding -4, desired induced draft pressure may be secured. adapted to be connected to ,buses` I5 and I6 and Counterclockwise rotation of the floating lever` the excitation ofwhich may be eontrolledby a 1I about the pivot point 36 moves the pivotally motor-operated rheostat 5 operated by a split associated link 3| upwardly, thereby moving the 40` series motor 6. `To control the direction of rotacontact-carrying lever 32 in a clockwise direction, tionoflthe motor 6,'a system 1 vof levers and pisand contact `members 33 and 34 are caused to tons is provided, whichlevers may be actuated engage. When the contact members 33 and 34 in `responseto `the pressure, or thequantity, of engage, a circuit is established from the bus I5 the fuel being supplied to the furnace and the through conductor 35, contact members 33 and 45 pressure ofthe-draft `induced by the motor 3. 34, conductor 36, resistor 31, and actuating coil In ordethat the motor 6 may not effect, at 38of the line contacter 9 to the bus I6. one operation, `the `complete:correction that may Operation of the line contactor Simmediately be desired for anychange inthe quantity of fuel closes the contact members 39 and 40 and places supplied to the furnace, a pairof line eontactors spring 5I under tension. Since the contact 50 8 and 9 are provided, which contactors control a members 52 have some inertiathe spring 5I perpair of inductive time-limit contactors I6 and I I. mits a short delay after the closing of the contact l Line contactors ftand 9 also preferably` have an members 39 and 40 before contactmembers 52 inductive time delay effect. These inductive open. Closing of the contact members 40 estab. time-limit contactors IIJ and I I, initurn, control lishes a circuit from the bus I5 through resistor 55 4|, signal 42, and contact members 40 to the bus I6. Signal may be a light of a specific color or a bell or any other signal having special characteristics distinct from the characteristics of the signal 66,. Signal 42 will indicate that line contactor 6 has operated and that the operation of the motor 6 is in such a direction as to increase the speed of motor 3, thereby eifecting a faster feeding of the air to the furnace.

The circuit .for the motor 6 may be traced from the energized conductor 35 through contact members 33 and 34, eld winding 43 and armature 44 of motor 6, conductors ,45 and 46, and contact member 36 to the bus I6.

The opening of the contact members 62 establishes a circuit from the'energized conductor 36 through current-limiting resistor 56, and magnetizing coil 53 of the inductive time-limit, contactor II to the bus I6. The time-limit contactor Il` is provided with a low resistance closed circuit 55, a neutralizing coil 22' and an adjustable spring assembly 56. The neutralizing coil 22 is connected directly across buses I5 and I6 through an adjustable current-limiting resistor 2l. By appropriate adjustment of resistor 2i and spring assembly 56, the time constant of contactor II may be changed to any selected value within a certain range.

Energization of coil 53 closes the contact members 54, thereby establishing a shunt circuit for the actuating coil 36 to deenergize this' coil. When coil 33 is deenergized, contact members 36 and 46 immediately open and contact members 52 close deenergizing the magnetizing coil 53. I'Iowever, since the inductive time-limit contactor II has a deilnite predetermined inductive time constant, contact members 54 will remain closed for a predetermined interval of time, so that motor 6 will not be immediately reenergized. In fact, the adjustment is such for the line contactor 9 and the time-limit contactor II, that a single operation of the motor 6 is not likely to effect a complete correction unless the change in fuel supply be very small. 'I'he result is that the rheostat 5 will be operated over acomparatively small range and the motor 6 will be deenergized by the opening of the contact member 36Ifor an interval of time determined by the time-limit contactor il, whereupon it is again reenergized by reason of the fact that the contact members 54 open after the lapse of said predetermined interval of time.

When the motor 6 operates as heretoforeex plained, more resistor sections are inserted in the field circuit, which circuit may be traced from the bus I5 through the field 4, conductor I1, re-

Y sistor sections I6 and conductor I9 to bus I6.

This intermittent operation of motor 6 permits the motor 3 to be stabilized vin its operation after each operation of motor 6 before motor 6 again operates. O r, more specifically stated, motor .6 operates for a comparatively short interval of time and is then at rest for a predetermined interval of time, which predetermined interval of time is usually longer than the operat# ing time of the motor 6. These alternate periods of operation and rest continue until the desired total speed change for motor 3 has been accomplished. Furthermore, after each change of rheostat 5, the motor 3 accelerates or decelerates, as the case may be, and by reason of the period of rest, motor 3 attains a stable operation before a second operation of motor 6. The speed of motor 3 is thus changed intermittently in small increments with the result that neither4 will the motor 6 over-correct nor will motor 3 temporarily overshoot.

With the rise in speed of the motor 3, the pressure in chamber 41 increases and in consequence the pressure in the upper portion of the cylinder 43 increases, thus forcing the piston 49 downwardly so that the pivot point 30, heretofore stationary, is moved downwardly. After repeated incremental changes in the resistance value in the circuit of the eld 4 of the motor 3, the speed will in time have changed suiliciently and, in consequence, the pressure on the discharge side of the induced draft fan, on the side of ilue 14,

' will have risen sufficiently to move the pivot point down by an amount proportional to the amount the pivot point 26 was previously moved upwardly, with the result that the arm 32 will take its neutral po'sition, as shown in the figure.

When it is necessary to decrease the induced draft pressure, as would be the case when the fuel supply is decreasedpivot point 29 will move down and the contact members 33 and 51 will engage, thereby establishing a circuit from ener'- gized conductor 35 through contact members 33 and 51, conductor 53, resistor 56, and actuating coil 66 of the line contactor 6 to the bus I6. Operation of the line contactor 3 will close the contact members 6I and 62 and, after a short interval of time, open the contact members 63.

The closing of contact members 62A energizes the signal 68 to indicate that the admission of fuel to the furnace is to be slower. Closing of contact members 6I establishes va circuit for motor 6 from the energized conductor 35 through contact members 33 to ,51, field winding 16, the armature 44 of the motor 6, conductor 45, and contactl member 6I to the bus I6. Whenthe contact members 63 are opened, the magnetizing coil 65 ofthe time-limit contactor' I0 is energized which thereupon closes the contact member 69 to deenergize the line contactor 6.

'Ihe time-limit contactor I6 is similar to the time-limit contactor II, also having a neutralizing coil 24 energized from the buses I5 and I6 through adjustable current-limiting resistor 23, a low resistance closed,circuit '66 and an adjustable spring assembly 61.l By suitable adjustment of resistor 23 and assembly 61 the time constant may be predetermined within a suitable ran e.

It will be noted that coil 65 is energized o iy a short interval of time after the operation of the line contactor 6, whereupon the contact members 69 are closed, thus providing, for a comparatively short interval of time, for the operation of motor 6. In fact, in some instances, motor 6 may not make even a single complete revolution but, nevertheless, eifects a small change in the resistance value in the circuit of the field 4, thereby decreasing the speed of the motor 3 by a small amount.

As soon as the time-limit contactor III has dropped, opening the circuit at 69,- line contactor 6 is again operated, if the correction is not sumcient, and this intermittent operation of the motor 6 continues until pivot point 36 has moved upwardly by an amount proportional to the, previous downward movement of pivot point 29,

` ing the pressure of the fuel being supplied to the hereinbefore. I desire. therefore, that my invention be limited only by the pertinent prior art and the scope of the claims appended hereto.

I claim as my invention:

1. In" a system for controlling Vthe operation of a furnace burning a fluidfuel and having an artificial draft, in combination, means for varyl mittentiy affecting the operation of thelmeans for controllingthe draft on the furnace. time limit means for controlling the time interval of each of said intermittent operations ofthe means for controlling the draft whereby said differential means produces its effect in small steps taken at predetermined vintervals of time.

2. In a system for controlling the operation of f Vze.'

a furnace burning a iluid fuel and having an ar- V`tiiicial draft. in combination, a motor having a field iwinding, a tan, for producing a draft in the furnace, driven by said motor. an adjustable resistor for controlling the speed of the motor and thus the pressure at the high pressure side of the fan, a `second motor for adjusting theresistanoe value of said resistor, means for varying the presv sure of the fluid fuel being supplied to the fur- `nace,differential means responsive to the difference of the pressure produced by the means for varying the pressure of the fuel being supplied to the furnace and the pressure at the high pressure side of the fan for controlling the direction Y of rotation of the second motor, to decrease the effect on the differential means thereby effecting a change in draft to 'correspond to the changes in the pressure of the fuel being supplied to the furnace, and time limit means, coasting with said differential means. adapted to energize thesecond motor by small steps taking place after the lapse of successive substantially predetermined intervals of time.

4EDWARD M. cLsY'roR. 

